Split secret detection into default and non-default branches jobs

What does this MR do?

This MR changes the default secret_detection job to scan commits on non-default branches as that is the common case. Users will be pushing up commits on feature branches and those commits should be scanned for secrets as they get pushed. The secret_detection_default_branch job runs as the old secret_detection job did -- it scans files at the HEAD of the default branch. Sample pipeline: https://gitlab.com/zrice/demo-historic-secrets/-/pipelines/165264461

Screenshots

Secret Detection Branch job:

Results:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team